1. Field of the Invention
The present invention relates to a gas-fired water heating apparatus, which comprises an upright cylindrical thermally insulated water tank, an axially extending exhaust gas pipe passing centrally through the water tank and a combustion chamber under the water tank, in which an atmospheric gas burner is arranged, which has a complete premixing fuel gas/air supply system.
2. Related Art
Gas-fired water heating apparatuses are marketed in many embodiments for different applications. They are used for providing hot water in living quarters or a home. A typical unit for example is the so-called boiler for a hot water supply.
The present invention especially relates to a special gas-fired water heating apparatus, the so-called “water heater” marketed in the USA. This water heater is widely distributed in the USA and is used, for example, in households, workplaces and small industrial operations to provide a free-standing hot water supply for supplying hot water, for example for a shower.
This “water heater” has a typical structure, as described in U.S. Pat. No. 4,953,510. The central component of the water heater is a cylindrical, longitudinally extended storage tank, through which an exhaust pipe extends and which stands freely on the ground on feet.
An atmospheric gas burner with suitable power is arranged in a combustion chamber under the storage tank, whose lower wall is cup-shaped. The hot exhaust gas including the combustion products from the gas burner acts on the bottom of the storage tank and subsequently rises through an elongated exhaust pipe. In this type of water heater the hot gas flowing upward in the exhaust pipe comes into contact with the interior surfaces of the exhaust pipe, while the water in the storage tank is in contact with the outer surfaces of the exhaust pipe. While the combustion proceeds, the water within the water tank is heated by heat conduction through the wall of the exhaust flue. In this type of water heater normally a longitudinal separating wall (baffle) is arranged within the exhaust gas pipe in order to improve the efficiency of the heat transfer through the wall of the exhaust gas pipe from the hot combustion air to the water within the water tank. Furthermore the outer surfaces of the storage tank are typically covered with thermal insulation, in order to reduce heat loss from the water both during heating of the water and after the combustion process is halted.
Increasingly strict legal regulations regarding avoiding accidents, especially when combustible vapors arrive in the open combustion region, and in regard to improvement of exhaust gas content, have led to developments in the water heater field, which provide a nearly completely closed combustion (burner) chamber, in which a complete premixing gas burner, a so-called pre-mix burner, is arranged.
This sort of water heater is described in U.S. Pat. No. 5,875,739. The structure and arrangement of the prior art pre-mix burner in the closed combustion chamber is shown in the appended FIG. 1, which is taken from the above-mentioned U.S. patent. This figure is a cross-sectional view of a water heater with a water tank or storage tank 1 and a nearly closed combustion chamber 2, to which an exhaust gas pipe 3 is connected, which extends through the center of the water tank 1. The combustion chamber 2 and the storage tank 1 are insulated from the surroundings by thermal insulation 4. An atmospheric pre-mix burner 5 is arranged within the combustion chamber 2, which comprises a mixing chamber 6 and a spherical burner mat 7. A Venturi 8 is connected to the mixing chamber 6, which extends through the thermal insulation 4 until at the outer surface of the water heater. A nozzle 10 for supplying the fuel gas is arranged within the outer opening of the Venturi 8. Primary air is drawn through the outer entrance opening 9, while fuel gas from nozzle 10 flows into the Venturi 8. The primary air is then mixed by turbulence with the fuel gas in the mixing chamber 6.
Another embodiment of a water heater with a complete premixing atmospheric gas burner is described in U.S. Patent Application 2003/0111 023 A1. This known gas burner 5 has a planar rectangular burner mat.
Using an atmospheric complete pre-mixing gas burner in a water heating apparatus of the above-described type allows the gas burner to be placed centrally in the combustion space, i.e. the combustion chamber, in order to attain a uniform heat transfer in all directions or to all sides.
The exhaust gas flow is directed so that the hot exhaust gases, understood herein to be combustion air, flow more or less directly from the combustion chamber into the exhaust pipe, because of the existing typical structure of the combustion system in the above-described water heating apparatus with the central exhaust pipe, which is connected flush with the lower cup-shaped wall of the storage tank and extends upward from it.
This has the following consequences:
1. Heat transfer in the bottom region, usually characterized as the dome, is not complete; and
2. Hot exhaust gas flows though the “exhaust channel” at comparatively high speed, so that the efficiency is limited. It can be increased by the so-called “baffle” in the exhaust gas pipe, but that impairs the quality of the combustion in a disadvantageous way, since the exhaust gas ratio NOx/CO is poor.